TURNER, HAEN^E AND HUBBARD GLACIERS 97
The general recession from 1891 to 1913 is thought to be normal, perhaps due to -decreasing snow supply, perhaps slightly accentuated immediately after 1899 by the •earthquake shaking in that year. It is assumed that Turner Glacier has not yet been subjected to any profound spasmodic advance, such as some of the other glaciers of the region have experienced, but that the two periods of slight forward thrust between 1899 And 1901 and between 1905 and 1906 represent only the response to moderate thrusts from some of the tributaries overloaded through earthquake avalanching. A great spasmodic forward movement may yet be in store for this glacier; or the effect of the •earthquake may express itself only by similar repeated small advances.
It is unfortunate that none of the tidal glaciers of Yakutat Bay have so far responded to the earthquake shaking, as some of the glaciers which terminate on the land have done. We cannot, therefore, tell what is to be expected as a result of such an advance. We would infer, however, that when such an advance occurs it will be a notable one, for the tidal glaciers are already moving with such rapidity that they are broken by crevasses from side to side, and we would expect the addition of a vigorous thrust to such rapidly-moving glaciers to push their fronts rapidly forward. Opposing this, however, is the tidal condition of the glaciers, owing to which advance under a forward thrust would be reduced by iceberg formation, especially if a grounded glacier front were pushed far enough forward to float, or if the glacier terminus is already afloat. Another difference between the tidal and non-tidal glaciers worthy of mention in this connection is that an advance in the former could not be detected by crevassing alone, as it might be in a non-tidal glacier, because their surfaces are already crevassed as much as is possible. For evidence of change in such a glacier in response to the earthquake shaking one would have to depend upon actual advance of the front, and upon changes along the margin.
When one of the tidal glaciers does push forward it will be a matter of great interest to observe the nature of the change not only because of the differences in condition, as •compared with those advancing glaciers already observed, but also as an indication of the extent to which it is possible for these large, vigorous glaciers to advance by this cause. It may have a distinct bearing on the interpretation of earlier changes of great •extent to which the glaciers of this region have been subjected. That these glaciers have not yet advanced need not necessarily be interpreted as indication that they will not do so. They are all long glaciers, with many tributaries, and it is reasonable to •expect that a considerable period of time must elapse before they respond to the down-shaking of snow and ice in their remote reservoirs. This is perhaps less true of the Turner than of the Hubbard and Nunatak glaciers, but even the Turner is one of the longest and most vigorous of the glaciers of Yakutat Bay; indeed, judging from the discharge of icebergs from its front it is more vigorous than the Nunatak Glacier. It is of course possible that neither Turner nor any of the other great glaciers may ever advance notably, not because of failure to receive great additions of snow and ice to their reservoirs by avalanching, for this is inconceivable, but by reason of minor responses, due to thrusts from individual tributaries at different times, dissipating the energy of the thrust that in other glaciers has been concentrated in one great spasmodic advance. While such a condition Is a possibility, it is hardly probable and we feel warranted in predicting that in time the Turner and other tidal glaciers of the Yakutat Bay region will in all probability show in a .spectacular way the result of such an advance. It is to be hoped that when this does occur, some observer will be on hand to witness its nature and extent.